Modern vehicles are usually equipped with power (servo) steering systems that support the driver when steering. Conventional power steering systems include an actuating device, such as for example an electric motor or a hydraulic pump, via which a support torque is exerted on the steering system that reduces the steering forces that must be applied by the driver. The magnitude of the steering torque is a function of the current driving situation, in particular on the steering or steering wheel angle and the driving speed. The support torque is usually set by a control device according to a prespecified steering characteristic.
In addition to the above-described power steering systems which reduce the application of force during steering, other conventional steering systems apply an additional actuating torque to the steering system in particular driving situations in order to cause the driver to change the position of the steering wheel. The systems are also called driver steering assistance systems. These include for example an LKS (Lane Keeping Support) system that helps the driver keep the vehicle in the driving lane.
LKS systems usually include an environmental sensor, such as for example a video system, with which the course of the lane and the relative position of the vehicle in the lane may be determined, or with which possible obstacles may be recognized. In normal driving operation, these systems are not active as long as the movement path of the vehicle corresponds approximately to the course of the lane. In this case, only the servo torque is applied to the steering system. If, however, the movement path of the vehicle deviates too far from the optimal movement path determined by the course of the driving lane, or there is some other dangerous situation, the system produces an additional actuating torque that may be haptically detected by the driver and that instructs the driver as to how he should actuate the steering system in order to keep the vehicle in the lane, or to execute an evasive maneuver. If there is a too-small actuation of the steering wheel, the driver will for example sense a decrease in the steering forces, causing him to automatically turn the steering wheel more strongly into the curve. Given a too-strong steering wheel actuation, in contrast, the system will increase the steering forces acting on the steering wheel, so that the driver will move the steering wheel further towards the straight-ahead position if desired.
For each steering intervention of the driver steering assistance system, the characteristic driving behavior of the steering system is modified, i.e., the driver suddenly feels a different counterforce at the steering wheel. The different steering feeling is unfamiliar to the driver and may be disturbing, in particular if the driver does not wish to cooperate with the proposed steering movement.